Some cordless power devices have an integrated battery pack that rarely needs to be removed by the user. The battery pack remains in the device for both charge and discharge cycles. Cell phones are a typical example and have battery packs that are charged while installed in the phones. This is possible for two main reasons: the battery can usually provide sufficient power for the cell phone for a relatively long period of time such as one to two days, and the user is not severely inconvenienced by charging the battery while it is still in the phone. Cell phones when on are typically in the stand-by mode waiting for the user to engage in a call and are typically used in the call mode where the user is engaged in a phone call for only short periods of time. Since cell phones draw relatively little power when in the stand-by mode, the battery can typically provide sufficient power to power the cell phone for the one to two day period.
These reasons do not apply to other types of cordless power devices, such as power tools. These other types of cordless power devices are often used continuously or frequently for relatively long period of times, such as during a work shift, and may use more than one battery pack during that period. For example, a cordless saw or drill used on a construction site may be used throughout the work shift and may use multiple battery packs during the shift. Plugging such a cordless power tool into a charger during use would be inconvenient, obviating the advantages provided by a cordless power tool, and in some cases isn't possible due to the lack of AC mains power where the cordless power tool is being used. For these reasons, cordless power devices such as cordless power tools are not well suited for using integral batteries. These cordless power devices, particularly cordless power tools for professional use, therefore have removable battery packs so that used battery packs can be swapped with fresh battery packs during the work shift and the used battery packs placed in a charger for recharging.
The design of the interface system between the battery pack and the cordless power device is important. “Interface system” as used herein means the elements of the cordless power device and battery pack that cooperate when the battery is inserted and removed in the cordless power device, including the elements that releasably secure the battery pack in the cordless power tool and those that provide the electrical connection(s) between the battery pack, the cordless power device, and the charger.
The designs of interface systems presently used for cordless power devices that use battery packs, particularly cordless power tools, leave something to be desired with regard to ease of use, latching, and/or electrical connections. The two most common types of interface systems used in cordless power tools having battery packs are the “tower cell” design, which is used in the DEWALT® 7.2V–18V systems, and the “rail-style” design, which is used in the DEWALT® 24V system.
U.S. Pat. No. 4,871,629 issued Oct. 3, 1989 for “Latching Arrangement for Battery Packs” shows an example of a cordless power tool in which the “tower-cell” design interface system is used. A distinguishing characteristic of the “tower-cell” interface system is that the removable battery pack includes a tower extending from a main body or base in which at least one battery cell is typically disposed, with the remaining battery cells disposed in the main body. The “tower-cell” interface system has a number of advantages. They include ease of insertion and removal. The tower aligns the battery terminals to the tool to facilitate insertion. One handed insertion is both possible and convenient. The latch buttons are placed in a natural grip position, making them easy to actuate with one hand, and gravity aids in the removal of the battery pack. The advantages of the “tower-cell” interface system also include utilization of the dead space in the tool handle in that the “tower-cell” fills up space in the foot of the tool, making the overall battery pack size when it is inserted in the tool seem smaller and user familiarity in that the “tower-cell” interface system is a commonly used interface system in today's cordless power tools.
The “tower-cell” design interface system can have certain disadvantages. They include the possibility of thermal imbalance due to the “tower-cell” being separated from the larger cluster of cells in the main body of the battery pack and thus tends to be at a different temperature during operation of the tool. The “tower-cell” design also typically introduces extra manufacturing processes and components, thus increasing manufacturing complexity. In a number of products utilizing the “tower-cell” interface system the “tower-cell” connections in the battery pack have presented reliability concerns. The relatively small terminal block area of the “tower-cell” design tends to preclude adding additional terminals to the terminal block. Also, the entire weight of the battery pack is supported by the latches which secure the battery pack to the tool and the contact area of the latches to the tool tends to be small, placing a relatively high amount of stress on the latches.
U.S. Pat. No. 6,057,608 issued May 2, 2000 for a “Cordless Power Tool System” shows an example of a cordless power tool in which the “rail-style” design interface system is used. A distinguishing characteristic of the “rail-style” design is that the battery pack housing has rails that ride on rails or rail sections of the tool housing. The “rail-style” design interface system has a number of advantages. They include battery pack retention in that there are separate mechanisms for supporting the weight of the battery pack (e.g., rails) and securing the battery pack to the tool (e.g., latch). The relatively large terminal block area of the battery pack facilitates system expansion in that there is space to add additional terminals. The cells are all contained in the body of the battery pack and no one cell is located remote from the other cells so that there is better thermal balance among the respective cells. The “rail-style” design interface system has better manufacturing consistency than the “tower-cell” design interface system in that there is no need to deal with the additional manufacturing complexity added by needing to deal with the “tower-cell.” Since all the cells are connected in the same way, the “rail-style” design interface system does not have the reliability concerns due to the “tower-cell” connections that some products using the “tower-cell” design interface system have experienced.
The “rail-style” design interface system can have certain disadvantages. They include rail contamination which makes insertion and removal of the battery pack more difficult. Insertion and removal of the battery pack is awkward compared to the “tower-cell” design interface system in that two hands are typically required to insert or remove the battery pack. Also, pushing or pulling on the battery pack during insertion and removal creates a moment about the tool making it more difficult to hold the tool in place. The battery pack housing has a “clamshell” construction and the tolerances of the rail dimensions are difficult to hold in the assembly of the clamshell, making rail travel “sticky.” The foot of the tool has dead space in that there is no “tower-cell” that fills the space in the foot of the tool.